Several of telephony's features and services are based on the location of the telephone. For example, a public safety ("911") system directs aid to the caller at the location of the calling telephone. Such systems function without the caller having to identify the location (an important feature for people unfamiliar with the area or unable to speak to the public safety personnel). The location of a landline telephone is static; the location is simply the point where the telephone is connected to the network. Since the location is static, the location and the telephone number are therefore stored in a database for later use. In contrast, a mobile station by definition has a dynamic relationship with locations in its wireless network; therefore, determining its position is more difficult.
To locate a mobile station with a high degree of accuracy, measurements of the distances of the mobile station from at least three fixed points are needed. The distance measurements are used in a triangulation algorithm to calculate a location relative to the fixed points. In wireless networks based on the Global System for Mobile communication (GSM) standard, obtaining the distance measurement to make a location determination of a mobile station engaged in a call is relatively straightforward.
In a GSM wireless network, a mobile station's distance from fixed points is derived from the time the mobile station's radio signal takes to arrive at different base transceiver stations (BTS's). Since the locations of the BTS's are known, the location of the mobile station is calculated to a relatively high degree of accuracy. The time values are readily available because of the manner in which the GSM standard specifies multiplexing of mobile stations on a given radio frequency.
According to the GSM standard, a mobile station on a call shares a radio frequency with other mobile stations by broadcasting and receiving only within a time slot assigned by the serving BTS. Signal propagation delay generally causes the mobile station's broadcast to be received at the serving BTS after its assigned time slot. To synchronize the mobile station with the BTS, the serving BTS calculates and sends a "timing advance" (TA) value to each mobile station. The TA is the amount of time that the mobile station must broadcast in advance of its assigned time slot so that the BTS receives the transmission during the proper time slot. Thus the TA value is the difference between the time that the mobile station's signal is transmitted ant the time the signal actually arrives.
The TA value may be used to derive the distance between the BTS and the mobile station, because signal propagation delay is usually a function of distance. The speed of the radio signal (the speed of light) times the TA (propagation time) yields the distance between the BTS and the mobile station. The point where at least three distance measurements intersect is the point of transmission and thus the location of the mobile station. While this location determination procedure is well known, determining which BTS's to use to obtain the TA values from is sometimes not as simple.
One prior art method for selecting BTS's is to select the serving BTS first. Next, BTS's from the mobile station's handover candidate list are selected. A problem with this method, however, is that some of the candidate BTS's may be cosited, that is, located at or near the same location as the serving or a previously selected BTS, and thus unsuitable for position triangulation. Furthermore, BTS's on the handover candidate list that are connected to a different base station controller (BSC) than the serving BTS are unsuitable because using different BSC's requires an unacceptable increase in message traffic among the components of the wireless network. Therefore, it is difficult to find sufficient BTS's for gathering TA values to make a position determination of a mobile station when there are not enough suitable BTS's on the mobile station's handover candidate list.